forEach method Null safety
override
Applies action
to each key/value pair of the map.
Calling action
must not add or remove keys from the map.
Implementation
void forEach(void f(String key, String value)) {
for (var i = 0; true; i++) {
final key = _key(i);
if (key == null) return;
f(key, this[key]!);
}
}